Preparation and Characterization of Tubular Ceramic Support for Use in Transesterification Process

A tubular ceramic support intended for use in transesterification process was prepared using the polymeric foam replication method. Pieces of foam with pores 120 ppi in size were completely immersed in a 25wt% solid-loading of porcelain slurry. The dried samples were sintered for 2 hours to temperatures ranging from 1200°C to 1265°C, respectively. It was found that the optimum sintering temperature was 1250°C as the following values were obtained for each characteristic: 1.98 MPa compressive strength, 47.3% porosity, 2.773gcm density and 0.45mm pore size. XRD results show that the sintered ceramic consists of mullite and quartz.


Introduction
Porosity is a defect formed by interfacial reactions which causes a decrease in the mechanical properties of ceramic and brittle solids.However, porosity can also be used to add new functionality to a ceramic material for use in industries which utilize ceramic foams.Ceramic foams are often used as membranes, absorbents, kiln furniture, catalytic converters, insulation, biomedical devices and as core material for sandwich construction [1][2][3][4], as well as for water purification or filtration purposes [5].Ceramic foams exhibit a great range of remarkable properties, including low density, low thermal conductivity, high temperature stability and high resistance to chemical attacks.
Ceramic foams are used as catalyst support or catalyst carrier in purifying or filtration processes where the ceramic foam is applied as a coating on the catalyst.This allows it to become a substrate or support to the catalyst.This paper is focused on the subject of ceramic foams as a catalyst support in transesterification process.Transesterification is a process of removing all glycerol and fatty acids from vegetables with the presence of a catalyst.Sodium hydroxide and potassium hydroxide are examples of homogeneous base catalysts used commercially in the transesterification process in earlier days [6][7].This is because these materials are the most economically efficient and can be used under low temperature and low pressure environments [8].However, one of the major disadvantages of homogeneous catalysts is that they cannot be reused or regenerated, and are difficult to separate from the end product because the catalyst is consumed in the reaction [8].Studies have been carried out to overcome the disadvantages of homogeneous catalyst by synthesizing and developing solid catalysts for use in transesterification reaction.In order to create the solid catalyst, the catalyst has to be deposited on the intermediate layer of the membrane support.Thus, the bottom support, also known as the substrate, must be tough enough to withstand the stress over the entire membrane.This is the major reason why ceramic foam was introduced as a support in this process.
Generally, ceramic foam used as catalyst support should have open pore sizes ranging from 0.02 to 1.5 mm in order to simulate very small particle diameters.This also provides high activity with low diffusion resistance and high porosities from 40 to 85% which lead to low pressure drop at high flow [9].There are three shapes that have been used commercially: disc shape, plate shape and tubular shape.In this study, the tubular shape was chosen because it provides mechanical strength to the intermediate layer and membrane layer so that they are able to withstand the stress induced by the pressure applied over the whole catalyst support, as illustrated in Fig. 1.Polymeric catalyst support has been commercially used, but since the last decade, a great deal of research has been focused on the development of new types of inorganic catalyst support [10].Inorganic catalyst supports, such as ceramics, possess good characteristics, including long term stability at high temperatures, resistance to harsh environments, resistance to high pressure drops, inertness to microbiological degradation, easy to clean and easy catalytic activation.Inorganic catalyst supports have a multi-layer structure which includes the substrate (support), intermediate layer (catalyst support) and membrane layer (catalyst).In fact, preparation of tubular porous inorganic catalyst support using the extrusion method was reported to have used alumina and cordierite [10], natural zeolite [11] and kaolin [9] as starting materials.However, preparation of tubular porous ceramic via extrusion is difficult since shaping by extrusion is possible only if the paste has rheological properties close to those of clay [13].Therefore, preparation of tubular ceramic support via the polymeric sponge method has been carried out as an alternative to the extrusion method because of its advantages, for example, controllable pore size and complex ceramic shapes for different applications.In a previous report, the tubular ceramic support was obtained by extruding a mixture of kaolin and starch as an organic additive.The kaolinstarch supports had a porosity ratio of 46% and an average pore size of around 1.4 µm while the kaolin-doloma supports had a total porosity of about 51% [14].Similar results were reported by [15] where the tubular macro-porous cordierite supports by extrusion had a total porosity of 41.4% and average pore size of 18.0 µm.SiC tubes with an outer diameter of 6mm, an inner diameter of 4mm and lengths of over 500mm were formed by extrusion, with porosities of 45.7%, 42.1% and 40.4%, pore sizes of 0.19, 0.22 and 1.70µm, compressive strengths of 47, 62 and 110 MPa, respectively [16].Cordierite supports with an external diameter of 8mm and internal diameter of 4mm, had a total porosity of about 45% [10].
The aim of this research was to produce a multilayer tubular ceramic support using the polymeric foam replication method.No reports similar to this study could be found in the literature.The optimum sintering temperature was decided based on the compressive strength, porosity, density and phase's presentation in the ceramic support.

Experimental 2.1. Foam preparation
PU Foam with 120 pores per inches (ppi) (Con-Dee Enterprise, Malaysia) was cut in dimensions of 120mm x 140mm.25wt% solid-loading of porcelain mixture was prepared and continuously stirred for 3 hours.The PU foam was immersed and then squeezed manually to remove trapped air prior to the dipping process in the slurry.The wet foam containing the slurry was rolled together with a cylindrical stick as a core in order to form a tubular shape.The tubular foam with a diameter of 45mm and length of 140mm was dried in a Memert Oven at 110°C for 18 hours.The dried sample was sintered in a Nabertherm 1540 Chamber Furnace from room temperature to 500°C with a heating rate of 3°C/min and 1 hour holding time.Then, the sample was further sintered from 500°C to temperatures ranging from 1200, 1225, 1250 and 1265°C, respectively, at a heating rate of 5°C/min and 2 hours holding time.

2.2.Property measurement
The linear shrinkage of the tubular ceramic support was determined based on the dried and sintered dimensions.The morphology and pore sizes of the sintered tubular ceramic supports were measured using Variable Pressure Field Emission Scanning Electron Microscope (VPFESEM SUPRA 35 VP) Model ZEISS, while porosity and density were determined using the water immersion method or Archimedes method.The compressive strength of the sintered tubular ceramic support with an outer diameter of 36mm, length of 36mm and thickness of 10mm was measured using a Universal Test Machine Model 5982 with a crosshead speed of 0.5mm/min.

Characterization
The crystallinity and phases of the porcelain powder and the sintered porous samples were analyzed by XRD (Bruker D8 Advance).Scans were performed between 2 values of 10° to 90°.Morphological study was performed using Variable Pressure Field Emission Scanning Electron Microscope (VPFESEM SUPRA 35 VP) Model ZEISS.

Results
By optimizing the porcelain, water and different temperatures for sintering, perfect circular cross-sectional shaped tubular ceramic supports were successfully prepared.Fig. 2 shows the sintered samples, all with an outer diameter of 38mm, inner diameter of 21mm and lengths of over 120mm.Longer sizes and smaller diameters can also be prepared, if The linear shrinkage of tubular ceramic supports sintered to 1200, 1225, 1250 and 1265°C are shown in Fig. 3.The linear shrinkage values increased in the range of 13.01 to 16.67% from 1200 to 1250°C, and after 1250°C, its values decreased to 14.67%.The shrinkage became intensive with the increase of sintering temperature.The tubular ceramic supports were prepared in the same order, except for the sintering temperature.A substantial shrinkage occurred after heat treatment in each sample, which is consistent with literature findings [3].This suggests that the PU sponges were removed as the particles were initially packed loosely, approached and contacted.The removal of PU sponges left holes in the center of the webs.During the sintering process, these holes, which became the source of void, would move from the center towards the outside and the particles moved to the internal surface of the webs.The holes in the center of the webs became smaller and led to the shrinkage of foams.The shrinkage became intensive with the increase in sintering temperature.Fig. 3. Linear shrinkage of tubular ceramic supports sintered to at different temperatures.Fig. 4 shows the average pore size and percentage of porosity of tubular ceramic supports after sintering to different temperatures (1200, 1225, 1250 and 1265°C).Below 1250°C, the average pore sizes increased while the porosities decreased with the increase of sintering temperature.After 1250°C, the average pore sizes decreased and porosity increased slightly with the increase of sintering temperature.The porosities decreased with the increase of sintering temperature at 1200, 1225 and 1250°C, but the grains grew and had more contacting areas.The significance of grain growth was permitted with the values of pore sizes, as shown in Fig. 6.A visual of the increase of pore sizes can be seen in Fig. 5. Similar results were reported by previous researchers where a decrease in total porosity of samples occured when the sintering temperature was increased [17][18][19].Fig. 5 shows the density and compressive strength of tubular ceramic supports sintered to different temperatures (1200, 1225, 1250 and 1265°C).Both the density and compressive strength of tubular ceramic supports gradually increased as the sintered temperature increased to 1200, 1225 and 1250°C.It was established that the highest compressive strength, 1.980MPa, was also attributed to highest apparent density, 2.773gcm -3 , at 1250°C.The density and compressive strength of the tubular ceramic supports slightly decreased as the sintering temperature increased to 1265°C.The relationship between the compressive strength and density can be expressed with the following equation: (2) where σ fc is compressive strength, C is a geometric constant characteristic of the unit cell shape, σ fs is the web strength and δ relative density which can be stated in equation (2) where p is the porosity.The correlation between compression strength and porosity can be described in equation (3) where: σ fc = Cσ fs (1 -p) 3/2 (3) These equations are consistent with the results reported in this paper, where the porosity decreased with the increase of sintering temperature and the compressive strength increased with the increase of sintering temperature.As a result, the compressive strength gradually increased together with the density of the tubular ceramic supports as the sintered temperature increased to 1200, 1225 and 1250°C.X-ray diffraction was used to identify the phases formed, as shown in Fig. 6a and 6b.The XRD pattern in Fig. 6a shows that the unsintered porcelain powder contains quartz and kaolinite.Mullite and quartz are the two crystalline phases identified from the XRD of tubular ceramic supports sintered to 1200, 1225, 1250 and 1265°C, as shown in Fig. 6b.The sample sintered to 1200°C shows quartz as the major crystalline phase, while the proportion of mullite phase is quite low.In order to encourage the growth of mullite crystals, the same composition was fired to 1250°C and 1265°C.This produced an increase in mullite peak intensity.This is consistent with previous studies [14][15][16] which reported that increased temperatures in ceramic composition will result in higher mullite formation.
Morphological investigation carried out by VPFESEM revealed some microstructural features, including open pores and closed pores, as shown in Fig. 7.With increased sintering temperatures, the microstructure changed obviously where the pore sizes became bigger.The XRD patterns consisting of mullite and quartz peaks confirm the microstructure results and the formation of coarser mullite crystals at higher temperatures.As shown in Fig. 7a, the open pores and closed pores were found to be irregular.It can be clearly seen that the open pores are contributed to the high porosity of the ceramic supports.When the sintering temperature was increased, the open pores grew then came into contact with other pores and became closed pores, as shown in Fig. 7b, c and d.The amount of closed poses increased with increased sintering temperatures as discussed in previous studies [14][15][16].The shrinkage of holes and webs led to the decrease of porosity in samples sintered to temperatures 1200 to 1250°C.As reported in this paper, the porosity of samples decreased from 58.6% (1200°C) to 47.3% (1250°C).When the samples were sintered to 1265°C, the linear shrinkage decreased and led to the increase in porosity.XRD patterns and SEM images support the results of compressive strength and density tests, where the amount of closed pores increased as the sintering temperatures increased due to mullite formation.The improvement of compressive strength was achieved because the increase in the sizes of the grains increased amounts of contact areas.The increased addition of pores results in lower compressive strength since higher porosity is obtained.Tubular ceramic supports with compressive strength values of between 1-3 MPa are suitable for use in transesterification process.
The results of this study show that the suitable sintering temperature for the production of tubular ceramic support using porcelain as raw material via the polymeric sponge method is 1250°C.Based on XRD analysis, the mullite phase became more intense at the mentioned sintering temperature, resulting in higher mullite formation, thus improving its mechanical properties.However, as the sintering temperature increased to 1265°C, the porosities of samples gradually increased due to the growth of a few grains which was caused by overfiring, consequently decreasing the compressive strength of the tubular ceramic support.It seems clear that the compressive strength of the support structures was directly influenced by the porosity of the supports, since the higher the porosity, the lower the strength, as shown in Table I.
Tab. I. Mechanical properties of tubular ceramic supports.

Conclusions
In this study, it was shown that tubular ceramic foam formed as supports can be prepared via the polymeric sponge method using porcelain as raw material.In general, this work has achieved the characteristics required of ceramic foams for use as catalyst supports, where the open pore sizes are within the range of 0.1 to 0.5mm and porosities are high, ranging from 47% to 59%.As an application in the transesterification process, the tubular support is able to withstand temperatures up to 600°C.Based on the reported results, the conclusions can be summarized as follows: • porosity play an important role in the mechanical properties of tubular ceramic foam.• by adapting the sintering temperature, tubular ceramic supports with good mechanical properties can be produced.

Fig. 2 .
Fig. 2. Photograph of PU foam a) after drying and b) after sintering.

Fig. 4 .
Fig. 4. The average pore sizes and porosity of tubular ceramic supports sintered at different temperatures.